1. Field of the Invention
The present disclosure relates to an organic light emitting diode (OLED) display device, and more particularly, to an organic light emitting diode (OLED) display device having an improved display quality.
2. Discussion of the Related Art
Among various flat panel display devices (FPDs), an organic light emitting diode (OLED) display device has a relatively high brightness and a relatively low driving voltage. In addition, since the OLED display device has an emissive type emitting a light for itself, the OLED display device has a relatively high contrast ratio and a relatively thin profile. The OLED display device has an advantage in displaying moving images due to a response time of several microseconds. Further, the OLED display device has no limitation in a viewing angle and has stability even at a low temperature. Since the OLED display device is driven with a low voltage of direct current (DC) 5V to DC 15V, it is easy to design and fabricate a driving circuit. Moreover, since a deposition apparatus and an encapsulation apparatus are all that is needed for fabricating the OLED display device, the fabrication process for the OLED display device is very simple. As a result, the OLED display device has been widely used for information technology (IT) equipment, such as a television, a monitor and a mobile terminal.
FIG. 1 is a circuit diagram showing an active matrix type organic light emitting diode display device according to the related art. In FIG. 1, a pixel region P of an active matrix type organic light emitting diode (OLED) display device includes a switching thin film transistor STr, a driving thin film transistor DTr, a storage capacitor StgC, and a light emitting diode (LED) E. A gate line GL is disposed along a first direction, and a data line DL is disposed along a second direction crossing the first direction. The gate line GL and the data line DL cross each other to define the pixel region P. A power line PL is parallel to and spaced apart from one of the gate line GL and the data line DL.
The switching TFT STr is connected to the gate line GL and the data line DL, and the driving TFT DTr is electrically connected to the switching TFT STr. In addition, the driving TFT DTr is electrically connected to the LED E and the power line PL. For example, a first electrode of the LED E may be connected to a drain electrode of the driving TFT DTr and a second electrode of the LED E may be connected to the power line PL. As a result, a source voltage of the power line PL is transmitted to the LED E through the driving TFT DTr. The storage capacitor StgC is formed between a gate electrode and a source electrode of the driving TFT DTr.
When a gate signal is applied to the gate line GL, the switching TFT STr is turned on and a data signal of the data line DL is applied to the gate electrode of the driving TFT DTr. As a result, the driving TFT DTr is turned on and a light is emitted from the LED E. The grey level of the light emitted from the LED E is determined according to the intensity of a current flowing from the power line PL to the LED E through the driving TFT DTr. Since the storage capacitor StgC keeps the voltage of the gate electrode of the driving TFT DTr constant while the switching TFT STr is turned off, the constant current flows through the LED E during a frame even when the switching TFT STr is turned off.
The switching TFT STr and the driving TFT DTr may include a semiconductor layer of polycrystalline silicon. The semiconductor layer of polycrystalline silicon is formed through a step of forming an amorphous silicon layer and a step of crystallizing the amorphous silicon layer. The step of crystallizing the amorphous silicon may be performed by various crystallization methods such as a crystallization method using a heat or a crystallization method using a laser beam.
Among various crystallization methods, a sequential lateral solidification (SLS) method using a laser beam has become of interest. The SLS method takes advantage of the fact that grains of polycrystalline silicon grow laterally from the phase boundary between liquid silicon and solid silicon. The SLS method can increase the size of the grains by controlling the energy intensity of the laser beam and the irradiation range of the laser beam used to grow the grains.
However, since sizes of the grains of polycrystalline silicon by the SLS method are different according to a direction, properties of TFTs including a semiconductor layer of polycrystalline silicon by the SLS method are also different according to the direction. As a result, a resistance of the semiconductor layer and the property of the TFT are determined according to a direction of a main grain boundary and a direction of a channel of the TFT. When a direction of a grain growth is parallel to a direction of a channel of the TFT, the TFT has an excellent device property. When a direction of a grain growth is perpendicular to a direction of a channel of the TFT, the TFT has a deteriorated device property as compared with the TFT where a direction of a grain growth is parallel to a direction of a channel. However, the TFT has an improved uniformity in a device property as compared with the TFT where a direction of a grain growth is parallel to a direction of a channel.
Each pixel region of an OLED display device may include a switching TFT controlling a data signal and a driving TFT controlling a current of an LED. Accordingly, each pixel region of an OLED display device requires at least two TFTs and at least one storage capacitor for obtaining a holding property of the at least two TFTs. For the purpose of obtaining a uniform display quality, each pixel region of an OLED display device may include a plurality of TFTs.
However, since a property and a uniformity of a TFT are determined by a direction of a grain growth of polycrystalline silicon, an OLED display device may have deterioration such as a flicker due to difference in property of a TFT when the plurality of TFTs are disposed without consideration of a direction of a grain growth. As a result, a display quality of an OLED display device is reduced.